A signal receiver may have a front end which includes a hybrid transformer to convert an unbalanced input RF signal into a balanced output, and to inject a local signal into the input RF signal.
One example of a hybrid transformer arrangement is as shown in FIG. 4. In FIG. 4, reference character e.sub.i designates an input RF (radio frequency) signal source providing an RF signal. The RF signal is applied through a capacitor C.sub.1 to an RF tank circuit 1 consisting of a coil L.sub.1 and a capacitor C.sub.2, and then through a capacitor C.sub.3 to an input terminal I.sub.1 of a hybrid transformer 2 serving as a balun.
The RF signal applied to the hybrid transformer 2 is of an unbalanced input type. Another input terminal I.sub.2 of the hybrid transformer 2 is grounded through a parallel circuit consisting of a resistor R.sub.1 and a capacitor C.sub.4. A local signal oscillator 3 outputs a local signal L.sub.0, which is applied through a capacitor C.sub.5 to a predetermined internal circuit connecting point P.sub.1 of the transformer 2. As shown in FIG. 4, the circuit connecting point P.sub.1 is connected to the connecting point of two windings which are series-connected between the input terminals I.sub.1 and I.sub.2.
Impedance means 4 (or coil L.sub.2) is connected between the balanced output terminals O.sub.1 and O.sub.2 of the hybrid transformer. The impedance means 4 provides low impedance for an IF (intermediate frequency) signal, and high impedance for the above-described RF signal and local signal.
The balanced output terminals O.sub.1 and O.sub.2 of the transformer 2 are connected to the sources of FETs Q.sub.1 and Q.sub.2, respectively. The drains of the FETs are connected to a tuning circuit 5, and gates are grounded.
The tuning circuit 5 is a parallel circuit of a capacitor C.sub.6 and a coil L.sub.3, which is tuned to the IF signal. The coil L.sub.3 is electromagnetically coupled to a coil L.sub.4. The signal induced on the coil L.sub.4 is provided, as an IF output, at an output terminal IFout.
A supply voltage +B is applied to the center tap of the coil L.sub.3, serving as an operating voltage for the FETs Q.sub.1 and Q.sub.2 forming a mixer circuit. The center tap, to which the supply voltage +B is applied, is grounded through a bypass capacitor C7.
In the above-described device, the input RF signal is mixed with the local signal, so that the difference in frequency between those two signals is extracted by the tuning circuit, thus providing an IF signal.
In the above-described device, the hybrid transformer is employed to provide a balanced mixer circuit. For use in a VHF or UHF band, the hybrid transformer must be of a transmission line type hybrid using a ferrite core. The hybrid transformer of this type is intricate in construction and high in manufacturing cost because it is formed by winding a pair of windings on a so-called "eye core".